1. Field of the Invention
The invention relates to a device for measuring electric current and voltage in a power feeding circuit of a plasma. In this document, such a device will be referred to as a “probe”.
2. Discussion of Related Art
The uses of the invention relate to all of the plasma-assisted industrial processes employed within a plasma reactor. In particular, such processes include (though this list is not exhaustive):                plasma etching (used in particular in microelectronics or in the nanotechnology area),        deposition of layers assisted by plasma (used, for example, for the manufacture of flat liquid crystal screens, etc.), and        applications for which the plasma is used as a light source or as a device for the treatment of gaseous effluents in pollution control applications or even as a thermonuclear fusion reactor, etc.        
The invention also applies to measurement of the electric current and the voltage in a plasma reactor using one or more variable electric voltage or current sources.
For processes such as those mentioned above, the invention can be used to ascertain, in real time and without disrupting the execution of the process, the essential electrical properties or characteristics of the plasma (current and voltage, but also the phase offset between current and voltage, etc.), and thus allows the modification, in real time, of the properties or characteristics of the electrical sources employed in these processes, in order to alter the characteristics of the plasma.
Such a modification in real time can be used to perform real-time control by means of a non-disruptive diagnosis based on the electrical measurements, in order to prevent process drifting or runaway.
One use of the invention is the control of these processes using the electrical measurements supplied by the probe.
Presentation of a Plasma Reactor
Prior to the description of forms of implementation of the invention, the following is a presentation of some characteristics of one (non-limiting) example of a plasma reactor that can be employed in the context of the invention.
Plasma reactors can be used to coat a sample with a thin layer of material, to etch a sample by ionic bombardment, or more generally to change the structure or chemical composition of a surface.
A plasma reactor can also be used as a light source or as a device for the treatment of gaseous effluents in pollution control applications, or even as a thermonuclear fusion reactor.
FIG. 1 schematically represents, in cross section, an example of a plasma reactor to which the invention applies. This reactor can, for example, be of the radio-frequency (RF) excitation type by capacitive or inductive coupling.
Such a reactor includes an enclosure under vacuum 53. Close to a first wall 54 of this enclosure, on a substrate holder 55, is placed a sample 56 to be treated.
The sample 56 is in the general shape of a disk of which one surface is directed toward the interior of the enclosure 53 and constitutes the surface to be treated.
The enclosure 53 is filled with a gas at low pressure, of the order of a few tens to a few hundreds of millitorrs, for example (a few tens to a few hundreds of pascals). The gas is obtained from a source 57 to be injected into the enclosure of the reactor via a gas feed pipe 58, with the gas flow being regulated by a flowmeter 59.
When a gas mixture is used, several sources, flowmeters and feed pipes are used in parallel. The gas is evacuated from the enclosure 53 via an evacuation pipe 60 connected to a pumping system 61 composed of one or more vacuum pumps in series. The pumping rate in terms of volume is adjusted by means of a valve 62.
The pressure in the enclosure is controlled with the valve 62 and/or the flowmeter 59.
A plasma reactor can also function at atmospheric pressure or in a low vacuum (pressure of gas between a tenth of one atmosphere and an atmosphere). The treatment of gaseous effluents for pollution control applications is often conducted at these pressures.
This is also the case for the continuous treatment of a large surface such as the deposition of layers onto window panes or cleaning steel sheeting as it leaves a rolling mill.
Several means can be used to generate the plasma 63. For example, in a configuration described as “reactive ionic etching by capacitive coupling”, a radio-frequency voltage is applied to the substrate holder. It is also possible, as shown in FIG. 1, to generate the plasma 63 by means of a source 64 that is independent of the substrate holder 55.
This source 64 can be associated with a generator 65 for the following source types for example:                an electrode powered by a high-frequency generator (capacitive source),        an electrode powered by a low-frequency generator,        an electrode powered by voltage pulses delivered by a pulse generator,        a coil powered by a radio-frequency generator (inductive source), and        a microwave generator.        
Where appropriate, the last two of the above-identified source types, i.e., inductive and microwave, can be associated with the use of a static magnetic field. In the case of the use of a source that is independent of the substrate holder, the latter can be polarized by a radio-frequency source 66 to establish a self-polarization and thus to increase the impact energy of the ions on the surface to be treated.
When the plasma source is a radio-frequency source, the latter can, where appropriate, be polarized at a higher frequency than that applied to the substrate holder 55 with the aim of preferentially controlling the electron density.
When the plasma source is a radio-frequency source (HF, VHF or microwave), an impedance matching or matching circuit 67 is placed between the generator 65 and the plasma source 64. This circuit is connected to the generator 65 by a transmission line 68, generally coaxial, with a characteristic impedance of 50 ohms. An impedance matching circuit is used to prevent the reflection of electromagnetic energy to the source. This firstly allows the source to be protected and secondly allows the transfer of power to the plasma to be optimized. This circuit modifies the electrical impedance of the plasma source in order to render it equal to the characteristic impedance of the line 68. The transmission line 68 is said to be matched. The matching circuit 67 is connected to the plasma source 64 by a coaxial or radial transmission line 69. This line is not matched since the impedance of the plasma source is not equal to the characteristic impedance of the line 69.
When the substrate holder is powered by a radio-frequency source, a matching circuit 70 is inserted between the substrate holder and the source. The latter is connected to the matching circuit by matched coaxial transmission line 71 whose characteristic impedance is generally equal to 50 ohms. The output of the impedance circuit 70 is connected to the substrate holder by an unmatched radial or coaxial transmission line 72.
The plasma processes using a radio-frequency source most often use a frequency in the high-frequency area (HF band: 3 MHz-30 MHz). Within this range, the frequency most often used is 13.56 MHz.
The plasmas affected by the invention include chemically reactive plasmas (in which both chemical reaction and ionic bombardment can be used).
Just the reactivity of the gas or of the gas mixture injected into the enclosure is sometimes the only phenomenon employed. In general this reactivity is improved or even generated by the collisions of the electrons with neutral atoms or molecules, thus producing radicals, e.g., unstable chemical species which are absent in the gas without the presence of the electrons. These radicals, as well as the reactive ions, are responsible for the deposition or the etching. In the case of deposition, we speak of chemical deposition on the plasma-assisted vapor phase. This reactivity initiated by the electrons avoids the need for significant heating of the gas or of the substrate holder, which would damage the sample to be treated.
The rate of production of radicals by electron collisions is a function of the electron concentration. Likewise, the flow of charged particles (electrons and ions) arriving at and leaving the surface to be treated is proportional to the electron concentration. Chemical reactivity and ionic bombardment generally act in synergy in these plasmas.
The electron concentration and the flow of ions are proportional to the electric current in the plasma. The flow of ions and the energy of the ions bombarding the surface to be treated are proportional to the voltage applied to the substrate holder 55 or to the electrode 64 in the case of a capacitive coupling source.
In a process of deposition or etching by plasma, it is important to know the characteristics of the plasma in order to be able to control the execution of the process and its reproducibility, in particular to control the speed of deposition or etching in accordance with the thickness of the deposition or the depth of the etching desired.
After deposition or etching, all the surfaces (electrodes, walls, etc.) exposed to the plasma are coated with a deposit that has to be removed in order to treat a fresh sample. This cleaning stage is often effected by means of a plasma, making use of both chemical reactivity and ion bombardment.
Measurement of the current flowing in the plasma or of the voltage applied to the electrodes 55 or 64 is therefore a means of controlling the characteristics of the plasma without disrupting it. This measurement is performed during the process or during the cleaning, and is preferably effected on the unmatched transmission lines 69 and 72 in order to be performed as close as possible to the plasma. The measuring probe can also be located on the matched transmission lines 68 and 71 in order to measure the quality of the impedance matching and, where necessary, to change the characteristics of the impedance matching circuits 67 and 70, and to improve the degree of matching of the lines 68 and 71.
Measurement of the current and of the voltage can be associated with a device designed to measure the phase offset between the current and the voltage, in order to deduce the power dissipated in the plasma and the impedance of the plasma. These last two parameters, as well as the amplitudes of the voltage and current, are useful for controlling the correct operation of these processes and the stages for plasma cleaning of the reactors. They can be used where appropriate to control a feedback loop in order to prevent drifting or run-away of the process. The quality of this control is strongly dependent upon the performance of the probe used to measure the current and the voltage.
Note that the invention applies more particularly to plasmas that are excited by a variable source of electric current or of voltage, such as a sinusoidal or pulse-type voltage generator.
The invention more precisely finds particularly advantageous applications in such plasmas excited with a sinusoidal radio-frequency voltage at a frequency of between 1 MHz and 1 GHz.
The electrical impedance of a plasma depends on the current flowing in the plasma, and is said to be non-linear. One of the consequences of this non-linearity is that a plasma excited by an alternating voltage source of frequency f generates harmonics of this excitation voltage at frequencies that are a multiple of f. For example, for a plasma generated by a sinusoidal voltage at 13.56 MHz, sinusoidal components at 27.12 MHz, 40.68 MHz, 54.24 MHz, etc., appear in the voltage and current measurement signals.
In the course of an industrial process such as those mentioned above, measuring the changes of the amplitude of these harmonics with time, in addition to the amplitude of the fundamental frequency in the course of an industrial process, has broad applications.
Such measurement can in particular be used to detect the end of the etching by plasma of a dielectric layer on a microprocessor during its manufacture. Note that the amplitudes of these harmonics at frequencies 2f, 3f, 4f, etc. are far lower than the amplitude of the fundamental component f, and that it is therefore necessary to be able to isolate them from this fundamental component by filtering.
In addition, plasma processes using a radio frequency greater than 13.56 MHz, and particularly in the very high frequency areas (the VHF band in particular, namely 30 MHz-300 MHz) are becoming common.
At such frequencies, the voltage and current probes have to operate over a very wide frequency range, since the frequency difference between each harmonic of the fundamental frequency component is higher than in the case where the fundamental frequency used is lower (13.56 MHz, for example).
Most of the existing probes designed to work at 13.56 MHz are therefore not usable at VHF. It would therefore be advantageous to be in possession of a probe designed to operate over a wide frequency range.
In addition, the size of the plasma-assisted etching and deposition reactors used in industry also tend to grow in order to treat a larger number of devices in a single operation.
These large-sized reactors necessitate the use of higher electrical RF powers. The RF currents and voltages to be measured also increase.
The risks of heating, short-circuit and material breakdown also increase at these higher currents and voltages, and so it would be advantageous to reduce these risks, in particular in order to be able to measure currents and voltages of large magnitude.
As explained above, it is often desired to measure the current and the voltage on the electrical power feeding circuit of the plasma process.
It is also often desired to determine the phase offset between the current and the voltage in order to deduce from this the power dissipated in the plasma and the impedance of the latter.
The quality of the measurement of phase offset is strongly dependent upon the performance of the sensor employed to measure the current and the voltage. This measurement should be precise, since the variations of phase offset are often very small.
It is observed with known voltage and current probes that the phase offset measured between the current and the voltage is affected by an error (this error generally becoming greater as the current and voltage sensors of the probe are more distant from each other). It would naturally be desirable to eliminate this type of error.
The solution, which would consist of bringing to the same level the current and voltage sensors of a probe of previous design (such as that shown in FIG. 2) in order to attempt to get around this type of error, would also increase the risk of mutual interference and would result in a degradation of the frequency response. The working frequency range of the probe would then be reduced. It is therefore necessary with this known type of probe to find a compromise between the risk of mutual disruption, the degradation of the phase offset measurement, and the working frequency range.
As mentioned above, there already exist probes that are designed to measure the current and the voltage delivered to a plasma.
FIG. 2 thus presents, in longitudinal section, a probe 10 mounted on an electrically conducting coaxial transmission line 20 which includes an inner conductor 21 and an outer conductor 22 that surrounds the inner conductor.
The coaxial line 20 is connected:                by its two conductors to an impedance matching circuit (not shown in the figure) which is also connected to an RF alternating voltage source (or RF generator) which excites the plasma (connection by the part of the line at the top of the figure),        by its inner conductor, to a radio-frequency electrode 31 in the form of a solid disk—only the cross-section of this disk appears in the figure (connection by the part of the line at the bottom of the figure), and        by its outer conductor to a conducting lid 32 which is also in form of disk and located facing and distant from the electrode 31 so as to form a space 30 between the electrode and the lid. The lid 32 is also electrically conducting.        
The coaxial line 20 described above corresponds, for example, to line 69 or line 72 in FIG. 1. The radio-frequency electrode 31 corresponds, for example, to the substrate holder 55 or to the plasma source 64 of FIG. 1. The lid 32 corresponds, for example, to the enclosure 53 or to the wall 54 of the vacuum chamber of FIG. 1.
Between the RF generator and the matching circuit, the line is said to be matched. Between the matching circuit and the plasma, the line is said to be unmatched.
The space between the inner conductor and the outer conductor is electrically insulating—it can comprise or consist of a vacuum or be filled with a dielectric material.
The line is traversed by currents moving in opposite directions along the core 21 and the envelope 22. These currents are generated by the alternating voltage source which excites the plasma by means of the RF electrode 31 which is in contact with the plasma.
These currents reduce and change direction—while also remaining in opposite directions to each other—twice in each alternating voltage cycle.
Note that because of the skin effect, high-frequency currents (“high frequencies” as used herein referring to frequencies above 1 MHz) flow at the surface of the conducting elements in which they are traveling (core 21, envelope 22, electrode 31, lid 32, etc.) and opposite, that is on the outside of the core 21 and on the outside of the envelope 22.
The probe 10 includes means 11 to measure the voltage between the current traversing the line 10 and an earth or a ground connected to the outer conductor 22, and means 12 to measure the current in this current.
The means 11 for measuring the voltage include:                a conducting disk 110 placed close to the inner conductor 21 and connected to a conducting cable 111 which traverses the outer conductor 22, and        a second conducting cable 112, connected to the outer conductor 22.        
Measurement of the voltage V2 between the two cables 111 and 112 thus normally corresponds to the voltage that one wishes to measure.
However, a voltage measured between these two cables has certain limitations:                firstly, the response of such a voltage probe is restricted in frequency,        secondly the operation of the transmission line 20 is disrupted by the proximity of the disk 110 to the inner conductor 21, and        finally the line 20 is partially short-circuited by the conductor 110, which can cause material breakdown, thus restricting the measurable voltage range.        
The means 12 for measuring the current include a conducting loop 121 (or several loops in series) placed close to the inner conductor 21, one end of which is connected to ground or earth (connection to the outer conductor 22).
The inner conductor is traversed by the sinusoidal Iplasma current that one wishes to measure.
This current induces a sinusoidal and azimuthal magnetic field (B), which induces a voltage (or electromotive force) between the ends of the loop 121. This constitutes an indirect technique for measuring the current, since it uses the magnetic field induced by the current to be measured.
The potential difference V1 measured between ground or earth and the end 1210 of the loop which is not connected to ground or earth is in principle proportional to the first derivative of the current (Iplasma) in the line.
In practice however, the loop 121 is also coupled capacitively to the central conductor which can add to the voltage measured at the terminals of the loop, a voltage which is proportional to the voltage (Vplasma) between the two conductors of the line 20.
This constitutes an additional voltage component which renders the measurement of the current less precise, and also disrupts the measurement of the phase offset between the current and the voltage.
Loop 121 disrupts line 20, since it forms a partial short-circuit between the two conductors 21 and 22, possibly leading to material breakdown. In practice, the use of such a loop is therefore generally limited to powers below 10 kW.
Moreover, because of the large size of the loop, it is also difficult to place a voltage sensor V2 close by without the current and voltage sensors disrupting each other. It is then necessary to move these two sensors away from each other—which then introduces an error into measurement of the phase offset between the current and the voltage.
It is generally necessary to very accurately calibrate such a known probe, in order to allow for the characteristics (geometry, size, etc.) of the loop 121.
Existing probes currently found in the targeted field of use employ variants of the probe described above.
In addition, these probes all employ indirect measurement of the current since they use the magnetic field induced by the currents flowing in the line 20.
Different versions of known probes can allow one to overcome one or more of the above-described limitations, but never to overcome all of them. For the purposes of illustration, probes are described in U.S. Pat. No. 5,834,931, U.S. Pat. No. 5,808,415, and U.S. Pat. No. 6,501,285.
Thus existing probes seeking to measure, in real time, the current and the voltage delivered by an RF generator to a plasma have various limitations.